The present invention relates generally to a method for producing articles from an extrudable, expandable synthetic plastic composition, and more particularly to the production of resilient plastic foam bodies having a nesting or interlocking configuration for use as a free-flowing, loose-fill packing material.
Packing materials comprising a multiplicity of plastic foam bodies made from an extrudable, expandable thermoplastic material, such as a polystyrene composition, are well known. As disclosed, for example, in U.S. Pat. Re. No. 27,243, dated Dec. 14, 1971, and assigned to the assignee of the present invention, such expanded foam plastic packing materials have many advantages over conventional materials such as excelsior or shredded newspaper. Among others, the expanded foam plastic materials are resistant to fungus growth, do not readily absorb moisture, and have good heat and electrical insulation properties. In addition, the plastic bodies may be configured to pour freely and flow around an object placed in a shipping container, then to interlock and prevent settling of the packing material when compressed.
With packing materials in general, and expanded foam plastic loose-fill materials in particular, it is important that the product have a low bulk density so that the gross weight of a filled package, and consequently its shipping cost, is minimized. Reducing bulk density also reduces manufacturing costs, since less raw material is needed for a given volume of product. It is, therefore, a principal object of the present invention to provide a method for producing a loose-fill packing material having a very low bulk density.
A more specific object is to provide a method for producing an improved packing material comprised of expanded foam plastic bodies having a low bulk density-producing configuration.
Another object of the invention is to provide an improved method for producing low density, expanded foam plastic bodies which are readily pourable, but which interlock or nest together when compressed, to prevent settling.
Still another object of the invention is to provide a method for producing expanded foam plastic bodies which interlock or nest together in such a way that void spaces are created between them.
A number of methods for producing expanded synthetic plastic packing materials are known in the prior art. Such methods include those disclosed, for example, in U.S. Pat. No. 3,188,264 and above-cited U.S. Pat. Re. No. 27,243, wherein an expandable synthetic plastic composition, such as polystyrene, is extruded through a die orifice and allowed to expand into an elongate rod or strip. The expanded rod is drawn forward from the die by conveying apparatus, commonly called a "puller", which supports the flexible rod for a distance, then feeds it into a cutter where the rod is sliced into segments of a desired length. Finally, the segments are warped and/or expanded further by a steam atmosphere exposure to produce a loose-fill packing material.
The methods disclosed in these patents have a number of drawbacks, however. They include the necessity of providing a puller to support the extrudate, i.e., the rod or strip, as it expands and cools prior to cutting. Moreover, the puller's speed must be carefully controlled to match that of both the extruder and the cutter. Otherwise, product quality will vary. Cutting an already expanded plastic foam ruptures cells on either side of the cut, weakening the foam body thus produced. In addition, the broken cells limit the amount of expansion or warping which can be achieved in the subsequent steaming process. This results in a final product having an undesirably high bulk density. The detrimental effects of cell rupturing are particularly significant when thin sections or chips are produced, since the ruptures may extend through the entire thickness of such a chip. Cutting an already expanded foam also requires considerable mechanical power and, particularly with polystyrene foams, is hard on cutter blades. Wear and breakage of the blades force frequent blade changes, increasing production costs.
In U.S. Pat. No. 2,576,977, a method is disclosed for making free-flowing cellular pellets by cutting an extruded thermoplastic composition into globules or segments as it emerges from a die and before substantial expansion occurs, and thereafter allowing the cut-off segments to expand. The resultant product of the disclosed method consists of spheres and ovoids, body forms which are undesirable for use as a packing material because they lack any ability to interlock or nest together to prevent settling of a packed object. Moreover, the bulk density of a mass of foam spheres or ovoids is undesirably high for economical use as a packing material.
Accordingly, a further object of the invention is to provide an improved method for producing an expanded foam packing material which eliminates these and other drawbacks of prior art methods.